This invention relates to aircraft distance measuring equipment (DME), and more particularly to improvements in digital DME systems.
Conventionally, DME operation involves the transmission of a radio signal of specified character from an aircraft to a ground station. After specified delays and under known constraints at the ground station, a reply signal is radioed back to the aircraft. Once the return signal is identified, and known time delays are accounted for, the duration between transmission and reception is decoded to yield distance from the ground station to the aircraft. Many aircraft utilize the same ground stations at the same time, however, so the aircraft receiver must contain logic which speedily and reliably discriminates the response to its own transmitted signal from responses which are being sent out to other aircraft.
In U.S. Pat. No. 3,781,888 to John W. Bail, assigned to the assignee hereof, there is described a digital DME system in which this logical discrimination process is accomplished quickly and effectively utilizing digital logic techniques. In that patent, a one-thousand bit shift register has bins corresponding to two-tenth mile increments from the aircraft to the ground station. The register is clocked corresponding to passage of the transmitted and received radio waves through two-tenth mile increments. A full shifting cycle of such a register is referred to herein as an "epic". During a first epic, all received signals are inserted into the register in timed relationship with each other and with transmitted signals (i.e., "pass" mode), and during subsequent epics the number of signals in the register is reduced by reinstating only those which are coincident with further received signals (i.e., "recirculate" mode). When but a single bin of the register is filled, its time relationship is translated into distance between the aircraft and the ground station, to the nearest one-tenth mile.
The apparatus and methods set forth in that patent afford excellent DME ranging capabilities, with the identification of a return pulse being accomplished with great speed and accuracy. Its operation may occasionally be frustrated, however, by failure of the ground station, for any of a variety of reasons, to respond during a given epic. That is, since the identification and generation at the aircraft of the valid distance pulse depends on receipt of the decoded pulse, failure to receive such a pulse during an epic causes the pass and recirculate cycles to be re-executed even though the valid aircraft to station distance (within the degree of precision of the system) has not changed.
A ground station may fail to respond to DME ranging signals from an aircraft for a variety of reasons. When a particular signal is received by the ground station, all further signals within a prescribed time (typically 100 microseconds) are ignored in order to avoid responding to echoes of the same signal which it has just received. Hence, any other signal arriving during that time period simply will not be favored with a response. Further, the efficiency of a ground station response is directly dependent on the number of aircraft currently utilizing it. Normally, a response can be expected 70 percent of the time, but during high traffic periods, the reply efficiency of the ground station may even fall to 50 percent. During such time, the station has a built in preference for higher powered signals, thereby substantially impairing the response efficiency to the lower powered signals. Since lower power may result from the length of distance between the aircraft and the ground station, or simply because of the power capability of the aircraft system (DME systems for commercial airliners involve substantially more power than those for small or private planes), it would be useful for the aircraft to be able to account and compensate for reduced reply efficiency from the ground station.
It is accordingly a primary object of the present invention to provide apparatus and methods whereby digital DME systems such as set forth in the above captioned patent are reasonably protected from having their own ranging capabilities diminished due to reduced ground station reply efficiency.